Brushless motors are well known. Conventionally, these motors comprise a housing rotatably supporting a rotor carrying a plurality of permanent magnets arranged in pole pairs. These magnets supply the field flux. A plurality of stators are are arranged about the rotor. Each stator has stator windings such that the interaction of current passing through the winding with the flux of the rotor magnets produces torque if the current through each stator winding is timed correctly with respect to rotor position.
Typically, brushless motors provide the necessary commutation position feedback information to a motor controller through the use of either an optical encoder mounted on the end of the motor or through Hall effect sensors placed in close proximity to a disc having a magnetic pattern thereon which rotates with the rotor.
The details of a brushless motor of this type using Hall-effect sensors for commutation information are described, for example, in U.S. Pat. No. 4,988,905 assigned to the owner of the present application.
In such brushless motors there is a necessity for decoding the output of the Hall-effect sensors in order to commutate the current through the various stator windings at the appropriate instant. U.S. Pat. No. 5,079,487 to Malang suggests that the motor drive can be commutated through use of a Programmable Logic Device. The device of this reference is constructed utilizing transistor technology and it is further suggested that MOSFETS may be substituted. The teaching of Malang is that the "back driving" of the power supply may be avoided by appropriate selection of the states of the regulating transistors.
There is associated with the use of MOSFETS a problem not recognized by Malang. Since high currents are being switched in the outputs FETS, if one of the two devices is not turned completely off before its complementary device is switched on, potentially destructive currents can exist.
U.S. Pat. No. 4,5644,868 also teaches the use of a programmable logic array to energize the windings. In this reference pulse-width modulation circuit monitors the current supplied to the motor and interrupts motor energization whenever the current rises above a reference level. The circuit is also used to provide regenerative braking whenever the direction of motor rotation is to be reversed. This reference also fails to address the problems with using FETS.